1. Field of the Invention
This invention relates to polybenzazole (PBZ) fibers and processes for the preparation of such fibers.
2. Description of Related Art
Fibers prepared from polybenzazole (PBZ) polymers may be prepared by first extruding or spinning a solution of the polymer in a solvent acid which is called a polymer “dope”, through a die or spinneret to prepare or spin a dope filament. The dope filament is then drawn across an air gap, with or without stretching, and then coagulated in a bath comprising water or a mixture of water and the solvent acid. If multiple fibers are extruded simultaneously, they may then be combined into a multifilament yarn during or after the coagulation step. The fiber or yarn is then washed to remove most of the solvent acid, and then dried. The physical properties of such fibers and yarns, such as tensile strength, are known to be relatively high.
Polybenzazole polymers and products made therefrom, including fibers and yarns, and methods of their manufacture have been disclosed in, for example, U.S. Pat. No. 4,533,693 (to Wolfe et al. on Aug. 6, 1985), U.S. Pat. No. 4,703,103 (to Wolfe et al. on Oct. 27, 1987), U.S. Pat. No. 5,089,591 (to Gregory et al. on Feb. 18, 1992), U.S. Pat. No. 4,772,678 (Sybert et al. on Sep. 20, 1988), U.S. Pat. No. 4,847,350 (to Harris et al. on Aug. 11, 1992), and U.S. Pat. No. 5,276,128 (to Rosenberg et al. on Jan. 4, 1994). Well-known polybenzazoles are polybenzoxazole (PBO), polybenzthiazole (PBT), and polybenzimidazole (PBI).
PBO spun from a solution of polyphosphoric acid has been found to lose its tensile strength in hot humid air. PBO loses as much as 40% of its strength in 80 days in humid air at 80° C. See the ZYLON® Technical Information Bulletin, revised September 2001 published by Toyobo Co., Ltd. This shortens the useful life of life protection articles, such as bullet proof vests, made from PBO.
U.S. Pat. No. 5,525,638 (to Sen et al. on Jun. 11, 1996) discloses a process for washing polyphosphoric acid from the polybenzazole dope filament to improve the initial tensile strength of the fiber or yarn as well as improve retention of tensile strength of the fiber or yarn following exposure to light and/or high temperatures.
JP2004076214 (to Tadao Kuroki, assigned to Toyobo and published Mar. 11, 2004) discloses a process to improve the strength retention of polybenzazole fiber after exposure to high temperature and high humidity for an extended time period. The final fiber contains a basic organic compound, in the form of a monomer or condensate of the monomer, selected from p-phenylenediamine, m-phenylenediamine and mixtures thereof. The basic organic compound is added to the fiber using a guide oiling method, showering method, or dipping method to fill voids in the fiber before the fiber is dried. The publication explains that the basic organic compound fills the voids of the fiber, thus, external steam is less likely to reach the polybenzazole molecules when the fiber is exposed to high temperature and high humidity for an extended period of time. It further explains that solvent remains in the polybenzazole fiber after the fiber is dried and such non-removed solvent is later neutralized by the base thereby reducing the loss of fiber strength with time.
JP2004076213 (to Tadao Kuroki, assigned to Toyobo and published Mar. 11, 2004) discloses a process to improve the strength retention of polybenzazole fiber after exposure to high temperature and high humidity for an extended time period. This process adds an organic pigment with high heat resistance and a thermal decomposition temperature of 200 degrees C. or higher to fill fiber voids anytime during or after the polymerization of the polymer. Like the prior Japanese publication, this publication explains that the basic organic compound fills the voids of the fiber, thus, external steam is less likely to reach the polybenzazole molecules when the fiber is exposed to high temperature and high humidity for an extended period of time. It further explains that solvent remains in the polybenzazole fiber after the fiber is dried and such non-removed solvent is later neutralized by the organic pigment thereby reducing the loss of fiber strength with time.
However, further improvement is desirable in maintaining the strength of dried polybenzazole fibers.
These and other objects of the invention will be clear from the following description.